1. Field of the Invention
The present invention relates generally to a multi-hop relay Broadband Wireless Access (BWA) communication system, and in particular, to an apparatus and method for providing synchronization channels to Mobile Stations (MSs) and Relay Stations (RSs) and eliminating near-far interference between a direct service and a relay service in a multi-hop relay BWA communication system.
2. Description of the Related Art
One of the most critical requirements for deployment of a 4th Generation (4G) communication system is to build a self-configurable wireless network. The self-configurable wireless network refers to a wireless network configured in an autonomous or distributed manner without control of a central system to provide mobile communication services. For the 4G communication system, cells of very small radiuses are defined for the purpose of enabling high-speed communications and accommodating a larger number of calls. Hence, a conventional centralized wireless network design is not viable. Rather, the wireless network should be built to be under distributed control and to actively cope with an environmental change like addition of new Base Stations (BSs). As a result, the 4G communication system requires the self-configurable wireless network.
For real deployment of the self-configurable wireless network, techniques used for an ad hoc network should be introduced to a wireless access communication system. Such a major example is a multi-hop relay BWA communication system configured by applying a multi-hop relay scheme used for the ad hoc network to a BWA network with fixed BSs.
In general, since a BS and an MS communicate with each other via a direct link, a highly reliable radio link can be established easily between them in the BWA communication system. However, due to the BSs being fixed, the configuration of a wireless network is not flexible, making it difficult to provide an efficient service in a radio environment experiencing a fluctuating traffic distribution and a great change in the number of required calls.
The above drawback can be overcome by a relay service that delivers data over multiple hops via a plurality of neighbor MSs or neighbor RSs. The use of the multi-hop relay scheme facilitates fast network reconfiguration adaptive to an environmental change and renders the overall wireless network operation efficient. Also, a radio channel in a better channel status can be provided to an MS by installing an RS between the BS and the MS and thus establishing a multi-hop relay path via the RS. In this way, high-speed data channels can be provided to MSs in a shadowing area or an area where communications with the BS are unavailable. Cell coverage can also be expanded.
FIG. 1 illustrates service provisioning in a typical multi-hop relay BWA communication system.
In FIG. 1, in the multi-hop relay BWA communication system, MSs 140 to 170 (MS1 to MS4) can receive the BWA services through a BS 100, a primary RS (RS1) 110, and secondary RSs (RS2) 120 and 130.
MS1 and MS2 within the service area 101 of the BS 100 communicate with the BS 100 via direct links L1. MS2, which is located at the cell boundary of the BS 100 and thus placed in a poor channel state, can receive a higher-speed data channel via an RS-MS link L2 between MS2 and RS2 130 than via the direct link L1.
MS3 and MS4 outside the service area 101 of the BS 100 communicate with the BS 100 via RS-MS links L3 provided by RSI 110. The communication links between the BS 100 and MS3 and MS4 via RS1 110 expand the cell coverage. MS4, which is located at the cell boundary of RS1 110 and thus placed in a poor channel state, can increase its transmission capacity using an RS-MS link L4 between MS4 and RS2 120.
As described above, when an MS is in a poor channel state at a cell boundary of a BS or in a shadowing area suffering from a severe shielding effect due to, for example, buildings, the BWA communication system enables the MS to communicate with the BS by providing a better-quality radio channel to the MS via an RS. In other words, the BS can provide high-speed data channels to the cell boundary and the shadowing area and expand its coverage area by the multi-hop relay scheme. The RSs 110, 120 and 130 are classified into RS1 (RS 110) that expands cell coverage and RS2 (the RSs 120 and 130) that increases capacity according to their relay capabilities.
Typically, transmission/reception is carried out between a BS and an MS in frames having the configuration illustrated in FIG. 2 in the BWA communication system. FIG. 2 illustrates a Time Division Duplex (TDD) frame structure compliant with Institute of Electrical and Electronics Engineers (IEEE) 802.16, for data transmission/reception between the BS and the MS.
In FIG. 2, a TDD frame 200 is divided into a DownLink (DL) subframe 210 and an UpLink (UL) subframe 220 with a guard region called Transmit/receive Transition Gap (TTG) in between. A guard region called Receive/transmit Transition Gap (RTG) is interposed between TDD frames.
The DL subframe 210 includes a preamble and a common control channel in mandatory slots. The MSs within the service area of the BS acquire synchronization and control information from the preamble and the common control channel.
As described above, the BWA communication system provides services to the MSs or RSs outside the cell coverage of the BS or in a shadowing area by use of the RSs. In order to ensure backward compatibility for the MSs, communications are conducted in frames configured as illustrated in FIG. 2. That is, an RS operates in the same manner as an MS during initial access and negotiates a relay operation with the BS so that BS can provide a relay service to MSs in frames having the configuration of FIG. 2. Because the RS provides the relay service using the same frame configuration as the BS, it has difficulty in concurrently communicating with the BS and the MSs over one frequency band in one frame. To avert a Radio Frequency (RF) isolation problem caused by the frame configuration illustrated in FIG. 2, the frames are configured as illustrated in FIG. 3 so that transmission to and reception from the RS occur in parallel in time.
FIG. 3 illustrates a TDD frame structure in a conventional multi-hop relay BWA communication system.
In FIG. 3, a DL subframe 300 is divided into a first area 301 and a second area 303, and a UL subframe 310 is divided into a first area 311 and a second area 313. For the RS operation transitions, the first areas 301 and 311 are distinguished from the second areas 311 and 313 in time division. The lengths of the first areas 301 and 311 and the lengths of the second areas 311 and 313 are fixed or adaptively adjusted according to a cell environment.
The BWA communication system provides a direct link service in the first areas 301 and 311 and a relay link service in the second areas 303 and 313. Hence, the BS provides a synchronization channel, a control channel, and a traffic channel to an MS connected to it by a direct link in the first areas 301 and 311 and a synchronization channel, a control channel, and a traffic channel to an RS in the second areas 303 and 313.
Since the RS may move as illustrated in FIG. 4, the BWA communication system should consider the mobility of the RS.
FIG. 4 illustrates movement of the RS in the conventional multi-hop relay BWA communication system.
In FIG. 4, being located in a vehicle such as a bus or a train, RS1 420 has mobility. Hence, the BWA communication system should provide a synchronization channel to RS1 420 for synchronization and cell search, taking into account its mobility.
In the case of the frame configuration illustrated in FIG. 3, the lengths of the first and second areas 301 and 303 of the DL subframe 300 may vary depending on a cell environment. The resulting change in the position of the synchronization channel at the start of the second area 303 imposes overhead because RS1 420 should locate the synchronization channels of the neighbor BSs. Increased interference between neighbor cells due to the power boost of synchronization channels, transmission of information about the neighbor BSs, and search for the synchronization channel of each neighbor BS add to the RS overhead.
Without providing the synchronization channels, RS2's 120 and 130, as illustrated in FIG. 1, provide the relay service in conjunction with the BS by multiple communications in the cell. In this case, an MS experiences near-far interference because of the power difference between a signal received from BS 100 or the RS1 110 and a signal received from the RS2 120 or 130, as illustrated in FIG. 5.
FIG. 5 illustrates a signal flow for a relay service from an RS in the conventional multi-hop relay BWA communication system.
In FIG. 5, within the cell area of a BS 500, a first MS 530 (MS1) in a good channel status receives a service from the BS 500 via a direct link and a second MS 520 (MS2) in a poor channel status receives the service via RS2 510.
Although BS 500 and RS2 510 perform multiple communications using orthogonal resources in the same time area, a BS link signal is overlaid with an RS link signal in the air. Thus, MS1 may undergo near-far interference as it receives a stronger interference signal from the nearby RS2 510 than a signal from BS 510. The near-far interference may also occur to the uplink as RS2 510 receives a stronger interference signal from MS1 than a signal from MS2.